1. Field of the Invention
The present invention relates to a charger for charging a secondary battery of, for example, a cellular phone, and an integrated circuit capable of controlling a charging operation for the secondary battery is charged.
2. Description of Related Art
In general, a secondary battery (secondary cell) has been used as a power source of a cellular phone etc. Referring now to FIG. 2, a conventional charger for charging the cellular phone etc. is described. As shown in FIG. 2, a conventional charger 101 includes a charging current source 110, and a charging control circuit 120. The charging current source 110 supplies charging current to the secondary battery 111. The charging control circuit 120 controls the charging current source 110. The charging current source 110 is composed of a charging transistor 112, a diode 114, a detection resistor 113, and the secondary battery 111, which are connected in series. The charging transistor 112 is connected to an adaptor voltage. A charging current I flowing through the charging transistor 112 is supplied to the secondary battery 111 to charge the secondary battery 111.
The charging control circuit 120 detects the voltage across the detection resistor 113 in accordance with the charging current I supplied to the secondary battery 111 from the charging transistor 112. Then, the charging control circuit 120 controls the charging transistor 112 based on the detection result. The charging control circuit 120 includes a detection circuit 130, a control circuit 140, and an operational amplifier 141. The detection circuit 130 detects the voltage across the detection resistor 113. The control circuit 140 outputs a control signal S1 in accordance with the detection result. The operational amplifier 141 controls the charging transistor 112 in response to the control signal S1.
The detection circuit 130 includes an operational amplifier 131. The operational amplifier 131 has a negative terminal (inverting input terminal) connected with one end of the detection resistor 113 by way of an external connection terminal 117 and a resistor 133. Meanwhile, a positive terminal (noninverting input terminal) of the operational amplifier 131 is connected with the other end of the detection resistor 113 by way of an external connection terminal 118 and a resistor 134. Further, a feedback resistor 132 is connected between the negative terminal and an output terminal of the operational amplifier 131. In addition, the positive terminal is also connected with a grounded resistor 135. The charging control circuit 120 controls the charging transistor 112 such that the voltage across the detection resistor 113 is kept constant.
In connection with such a circuit controlling a charging operation for the secondary battery, Japanese Unexamined Patent Application Publication No. 9-84276 discloses the following charging method. That is, at an initial stage of charging, a charging transistor is turned on so as to quickly charge a secondary battery in a short time. After the secondary battery is charged to the fullest, the charging transistor is turned off, and a transistor smaller than the charging transistor is turned on. This aims at preventing the secondary battery from being overcharged as a result of quick charging by the charging transistor. To that end, the aforementioned smaller transistor takes over the charging operation for the secondary battery with a minute current.
Incidentally, in keeping with recent trends that various functions are being added to the cellular phones, there is a growing need to increase the battery capacity. Along with this, there is a tendency to increase the charging current for shortening a charging period.
In the above conventional charging control circuit of FIG. 2, and the charger disclosed in Japanese Unexamined Patent Application Publication No. 9-84276, however, a larger amount of charging current needs to flow through a charging transistor for quick charging at the time of charging, with a view to shortening the charging period as well as increasing the charging capacity. Such a large current causes a problem that the heat generation of the charging transistor is increased.